The present disclosure relates to a power amplifier circuit. A power amplifier circuit to be incorporated in a mobile communication device such as a cellular phone typically includes a bipolar transistor as an amplifier. Bipolar transistors have a thermal positive feedback characteristic in which as the temperature of the element increases, collector current increases, which further raises the temperature of the element, leading to a further increase in collector current. To suppress an increase in collector current caused by an increase in temperature, a configuration is available in which, for example, a resistance element (hereinafter also referred to as a ballast resistor) is connected between a base of a bipolar transistor and a base bias voltage supply terminal. In this configuration, a voltage drop across the ballast resistor suppresses an increase in base current, and, as a result, an increase in collector current is also suppressed.
In a configuration including a ballast resistor, when base current increases with an increase in the power level of an input signal, voltage drop increases across the ballast resistor, which causes a decrease in base voltage. As a result, power gain may decrease with the amplitude of the collector current, which is independent of the amplitude of the base current, and the linearity of the amplifier may deteriorate. To prevent the deterioration of the linearity, for example, Japanese Unexamined Patent Application Publication No. 2003-324325 discloses a power amplifier including a capacitance element between a signal input terminal and a base bias voltage supply terminal. With this configuration, power supplied from the signal input terminal can be transmitted to the base bias voltage supply terminal. Thus, a reduction in base voltage is suppressed, and linearity is improved.
In the configuration disclosed in Japanese Unexamined Patent Application Publication No. 2003-324325, the capacitance element connected between the signal input terminal and the base bias voltage supply terminal may be part of a matching circuit viewed from the signal input terminal. That is, the capacitance element may affect matching between the input impedance of an amplifier and the output impedance of a circuit preceding the amplifier.